The present invention relates generally to a method and apparatus for imaging and analyzing objects using ultrasound. More specifically, the invention relates to a method and apparatus for real-time three-dimensional acoustoelectronic imaging, including both direct-imaging and acoustic holographic methods, to image and analyze tissue and tissue structures.
Because of certain disadvantages with other techniques, the medical community has looked to ultrasound to provide a safe, low-cost, high-resolution imaging tool. For example, imaging systems based on x-rays, including x-ray mammography and x-ray computed tomography (CT), are limited to providing images of pathologies within a body and do not characterize the features in an independently meaningful way. Techniques based on nuclear magnetic resonance, such as functional MRI, are more sophisticated and can provide physical characteristics of tissue, but they are slow and expensive, and therefore unsuitable for routine screening applications. Also, x-ray-based techniques use ionizing radiation, which is known to present certain health risks to patients. In addition to the relatively high expense associated with CT scans and MRI, the expertise of highly trained personnel is needed for extended periods of time to operate the devices and interpret the results. Perhaps most important, however, is the fact that these techniques rely exclusively on two-dimensional images, thereby disguising three-dimensional structure information that can be critical for diagnosis. Efforts have been made to produce pseudo-three-dimensional images (or 2.5-dimensional images) by collecting multiple discrete two-dimensional image slices, but these are not as valuable interpretively as a proper three-dimensional technique.
Acoustic waves propagate through inviscid liquids as longitudinal waves. Through viscous liquids and solids, acoustic waves may propagate as either longitudinal or shear waves and may be converted from one type to the other at boundaries between media of differing acoustic properties. The principal property determining the scattering of acoustic waves from these boundaries is the wave impedance xcfx81c, where xcfx81 is the density of the medium and c is the sound speed, which generally has complex values in attenuating media. Since acoustic waves scatter at all boundaries and since the details of the scattering contain information about the two media in contact where the scattering arises, it is an objective of diagnostic ultrasound to recover the information available from the scattering. B- and C-scans are limited to backscatter in this regard. Transmission techniques use mostly the forward scatter. It is well known that different information is derived from the scatter depending upon the scattering angle with respect to the initial sound at which it is received. Thus, it is desirable to receive as much scattering information as possible.
In conventional B- and C-scan ultrasound analysis, a small array of approximately 1000 elements is moved by hand in contact with the patient. The array sends out acoustic waves that reflect from tissues back to the same array. Trained technicians and physicians conduct the ultrasound imaging procedure and interpret the results. The technique is well known to be limited in application because ultrasonic B and C scans provide only reflectivity information, which is most significant at boundaries between different types of materials, without providing other information on the properties of the materials themselves. In particular, the reflectivity is determined by the ratio of wave impedance, which involves the properties of both media forming the boundary. A change in reflectivity produces the strong reflections used in B-scans. Furthermore, the only radiation that is used is that reflected back to the hand-held sensing array. Depending on the ultrasound frequency, resolution is approximately 1-2 mm at a depth of 2-3 cm and about 5-7 mm at a depth of 5-6 cm.
There thus remains a need in medical imaging for a noninvasive method and apparatus that captures and analyzes full-field through-transmission data to provide full three-dimensional imaging and characterization of tissues.
Embodiments of the present invention provide an acoustoelectronic method and apparatus for generating a real-time three-dimensional representation of an object. Such embodiments improve on the two-dimensional ultrasound shadowgram images of the prior art. In one embodiment, the three-dimensional representation is generated by direct acoustoelectronic imaging. A first electrical signal is generated and used to derive an incident acoustic signal that insonifies the object. Resulting acoustic signals scattered from the object are collected and converted to digital electronic signals at substantially a rate defined by the acoustic frequency of the acoustic signal. The digital electronic signals are each time-resolved into an amplitude and phase from which the three-dimensional representation of the object is then produced.
In one such direct-imaging embodiment, additional information for producing the three-dimensional representation of the object is obtained by comparing the amplitude and phase of a reference electrical signal with the time-resolved amplitudes and phases of the digital electronic signals. The reference electrical signal may be derived from the same first electrical signal used to derive the incident acoustic signal. The first electrical signal may be a shaped electrical pulse or a continuous-wave electrical signal in different embodiments. In various embodiments, the scattered acoustic signals are collected at a two-dimensional array of acoustic receivers. Each acoustic signal received at the array is independently digitized, thereby permitting parallelization that allows hundreds of scans of the object to be completed within the standard {fraction (1/30)}-s period used for video images.
Certain of the direct-imaging embodiments further include acoustic lenses. In one such embodiment, a transmitter acoustic lens is positioned so that the incident acoustic signal passes through the lens towards the object. Additional imaging information is acquired by also positioning a second transmitter acoustic lens through which an orthogonally directed incident acoustic signal passes towards the object. Such transmitter lenses may be moveable along the respective propagation directions to focus the acoustic signals at particular locations within the object. In a certain embodiment, a receiver acoustic lens is positioned to focus the scattered acoustic signals onto the acoustic receiver.
In further direct-imaging embodiments, the acoustic transmitter is a pixelated two-dimensional transducer array, which is suitable for near-field focal region scanning of the object. Scanning is achieved in one such embodiment by varying the frequency of the incident acoustic signal as emitted from different pixels of the transducer array. In an alternative embodiment, the amplitude of-the incident acoustic signal is varied for different pixels. In a particular embodiment, the transducer array is circular with independently driven radial ring separations and is configured successively to excite increasing numbers of such radial rings. In a related embodiment, the circular transducer array is sectioned radially and circumferentially and configured to quadratically vary the phase of the incident acoustic signal emitted from different pixels.
Other embodiments of the invention are directed to generating a real-time holographic representation of the object. A first electrical signal is generated and used to derive an incident acoustic signal to insonify the object. Scattered acoustic signals are then collected from the insonified object and converted to digital electronic signals for recording an electronic interference pattern. The representation of the object is obtained from the recorded pattern from a computational reconstruction of the wavefront by electronically diffracting a reference electrical signal with the recorded pattern. The reference electrical signal may be generated from the first electrical signal, which is a shaped electrical pulse or a continuous-wave electrical signal in different embodiments. The scattered acoustic signals are collected in one embodiment at a two-dimensional array of acoustic receivers. In further embodiments, multiple acoustic sources and multiple two-dimensional acoustic receiver arrays are used.